This invention relates to a method and system for the real-time measurement of aqueous cyanide and more particularly to method and system for the continuous, real-time measurement of cyanide using a quartz crystal microbalance (QCM).
There are numerous industrial processes that would benefit from a continuous on-line cyanide detection device that serves as a means for monitoring process performance and environmental compliance (i.e., downstream discharge levels). Such a device would find immediate applicability in leach-mining operations, electroplating facilities, hydrocarbon refining and chemical synthesis plants, environmental monitoring, and others.
The present invention overcomes many of the shortcomings experienced by conventional cyanide measurement techniques, as described below. The methods and procedures for conventional cyanide analytical methods suffer from two primary types of inadequacies. The first problem is the inability to obtain reliable results at concentrations low enough to be meaningful, i.e. the detection limit problem. This is a crucial issue since the detection limits for accepted methods of cyanide analysis often exceed or barely meet the EPA-defined cyanide level of 5 μg/L for freshwater and marine aquatic life.
The second area of inadequacy stems from the fact that a wide variety of chemical compounds commonly associated with natural waters and cyanide solutions can cause chemical interferences for the detection methods. Such chemical interferences can compromise accurate cyanide quantification and have a detrimental effect on a method's precision in determining detection limits. Both of these problems are significant barriers to the accurate monitoring of cyanides in mining processes.
Currently the difficulties tied to analytical detection limits for accepted methods are such that the EPA has set a regulation that cannot be rigorously and reliably attained. This places all those wishing to comply with the regulation in a “Catch-22” situation. In examining the detection methods we find that the titrimetric method has the highest limit of detection. The ASTM Procedure indicates that a skilled and experienced analyst can obtain reliable cyanide determinations at concentrations down to 1000 μg/L. This is nearly three orders of magnitude above the maximum allowable concentration of cyanide. The colorimetric and potentiometric methods provide significantly better sensitivity. The calorimetric method has been used to provide the bulk of the currently available environmental cyanide data. Utilizing guidelines provided by the EPA, the colorimetric method provides a detection limit of 20 μg/L, which is four times higher than the EPA limit, and this is only within a 10 percent error. The potentiometric determination technique has a lower limit of detection of approximately 26 μg/L, still five times that of the maximum allowable cyanide concentration. This leaves only the automated UV method, which in its procedure has stated a method limit of detection of 5 μg/L. This means a sample containing the maximum allowable concentration of cyanide produces a signal that is just discernable. Unfortunately, thiocyanate (SCN−) is also detected by this method and since very few cyanide samples exist without thiocyanate also present, and since there is no reliable method that determines thiocyanate without cyanide interfering, the UV method detection limit is also uncertain. To complicate these analyses further, each of these methods has a wide variety of other possible interferences, making the stated problems with detection limits somewhat ubiquitous among current methods.
Of the many chemical species that interfere with the cyanide detection methods, thiocyanate is by far the most serious. This is due in part to the fact that thiocyanate is often present in a large excess relative to the cyanide content of a sample. This is especially true for solutions produced by the mining industry, where cyanide quickly reacts with sulfides in the ore during the cyanidation process.
The same reaction occurs in natural waters containing sulfide and in the alkaline absorbers used in the distillation methods. During distillation, hydrogen sulfide distills with the newly created free cyanide and both are absorbed in the scrubber solution. Here, the cyanide, which was present as a complex, now freely reacts with the sulfide. This reaction is accelerated at elevated temperatures and pH, which are precisely the conditions present in the scrubber solution. This is a significant problem since thiocyanate reacts equally well as cyanide to chloramine-T, therefore allowing both ions to be colorized in the colorimetric procedure, making thiocyanate a major positive interference for the most widely used cyanide determination method. Potentiometric detectors are also affected by the thiocyanate ion. Utilizing some of the newest cyanide ion selective electrodes available, it is reported that a one hundred-fold excess of thiocyanate produces a positive interference equivalent to nearly three times the equivalent amount of cyanide present. Sulfide has also been shown to deactivate the surface of the cyanide electrodes when they are immersed in sulfide containing solutions. While this interference-deactivation translates to only a minor interference for distilled samples, it precludes the direct use of the ion selective electrode in natural water samples.
Thiocyanate is almost completely decomposed during the UV-digestion used in the automated analysis, leading to a large positive interference. The UV-digestion procedures require a separate analysis for thiocyanate, if present, in order to correct the total cyanide value. The methods of analysis commonly used for the determination of thiocyanate are colorimetric and are the same methods which are used for cyanide, the pyridine-pyralozone method and the copper-pyridine method. Therefore, cyanides are a serious interference in both methods. This illustrates that while the automated UV-digestion method reports the lowest detection limits among the currently accepted cyanide detection methods, the use of this method is severely limited by the thiocyanate interference. In summary, the thiocyanate ion is the root of much discourse in cyanide analyses, since it (a) interferes with all sensitive cyanide detection techniques, and (b) itself is not easily measured or completely maskable making the corrections for thiocyanate unreliable.
In a marked deviation from the standard methods of cyanide determinations, the piezoelectric detection method and system of the present invention is largely free from chemical interferences. It is seen that detector sensitivity is practically independent of the presence of common cations and anions. Perhaps the most important result shown in FIG. 6 is that thiocyanate interference is also extremely minimal, yet quantitative. As a result thiocyanate generally produces no performance limiting interference for QCM operation.
An advantage of the present invention is that it can provide continuous analysis, whereas the other methods require longer analysis times. This advantage provides for near-immediate process control for systems requiring adjustments related to a changing cyanide level (e.g., leach mining operations involving cyanide). Tighter process controls allow for a more economical use of chemicals and materials consumed by a given system.
It is therefore an object of this invention to provide a continuous, real-time aqueous cyanide detection method and system at very low detection levels (e.g., down to 1 μg/L) that is largely free from chemical interferences.
It is another object of this invention to provide a cyanide detection method and system that is essentially independent of the presence of cations and anions.
It is still another purpose of this invention to provide a cyanide detection method and system to quantify and minimize thiocyanate interference.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.